Angle modulated wave reproducing system



Dec. 3, 1946.' c. M. slNNET-r 2,412,015

ANGLE MODULATED WAVE REPRODUCING SYSTEM LIM lNvENToR Zw-fn? M JPM/V577:

ATTORNEY 3 Sheets-Sheet 2 Filed March 20. 1944 aa/a Mme'.

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INVNTOR (yf-fra?? M N/V577.'

ATTORNEY Dec. 3, 1946. v l c. M. slNNl-:TT 2,412,015 l ANGLE MODULATED WAVE vREPRODUCING SYSTEM l I Filed March 20, 1944 3 Sheets-Sheet 5 ATTORNEY Patented Dec. 3, 1946 ANGLE MODULATED WAVE REPRODUCING SYSTEM Chester M. Sinnett, Westmont, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 20, 1944, SerialNo.. 527,264

19 Claims.

My present invention relates generally to a novel method of, and systems for, reproduction of audible frequencies existing in the form of angle modulation on a superaudible wave.

In the past it has been proposed to reproduce sound records by translating the recorded variations into reactance variations, angle modulating a superaudible oscillator in accordance with the reactance variations, converting the angle modulated oscillations into corresponding amplitude modulated oscillations, and nally rectifying the latter and reproducing the audio frequency currents resulting from rectification. The generic term angle modulated is intended to include frequency modulation (FM) and phase modulation. The aforesaid converting and rectifying steps have been common to other sound reproducing methods and systems, whether the source of input energy was a record bearing impressions representative of audio signals or angle modulated superaudible oscillations. In the case of frequency modulation radio reception, as well, discrimination and rectifying steps are employed to provide modulation signals for nal reproduction.

A general object of my present invention is to provide a system for reproducing frequency modulated superaudible waves, wherein the. discriminator of the system has a reactive element thereof which functions as a modulation reproducer thereby eliminating the need for the conventional form of discriminator-rectifier network commonly employed in frequency modulation systems. v

According to another important object of my present invention, I utilize an electrostatic type of reproducer as an essential reactive element of a simple tuned circuit acting to convert angle modulated superaudible waves into corresponding amplitude modulated waves with concurrent acoustic reproduction of the modulation on the amplitude modulated waves.

Still another important object of my present invention is to provide a novel method 4of reproducing sound, wherein angle modulation of a superaudible wave is employed to represent the currents corresponding to the sounds to be reproduced by a transducer, and the transducer itself acts as an essential reactive component of a discriminator network for the angle modulated waves. y

A more specific object of my invention is vto provide a system of reproducing sound records, wherein a pickup is employed to convert the record impressions into capacity variations of a superaudible oscillator circuit thereby to provide frequency modulated superaudible oscillations; the essential improvement in the system including the replacement cf the usual discriminatorrectier network by a discriminator network whose condenser element acts as an electrostatic reproducer.

A further object of the invention may be stated to reside in the provision of a frequencyresponsive network, adapted to have angle modulated waves applied thereto, which consists of a coil and a condenser providing together a resonant circuit tuned off-resonance relative to the mean frequency of the applied waves, and either the coil or condenser acting as a reproducer element whereby the angle modulation on the applied waves is directly converted into sound.

A still further specific object of this invention is to provide a phonograph, or other sound reproducer device, operating onl the frequency modulation principle and employing a superaudible frequency modulated carrier, wherein the modulated carrier is amplified and fed into a tuned discriminator circuit, one reactive element4 of which functions as an electrostatic loudspeaker.

A further object of my present invention is to` provide a. novel method of operating a condenser type of sound reproducer, wherein the polarizing voltage of the reproducer is provided by superaudible wave energy applied to the reproducer.

Still other objects of my invention are to pro.- vide novel sound reproducer arrangements, whether condensive or magnetic, wherein angle modulated superaudible carrier waves are directly applied to the reproducer for acoustic reproduction of the modulation.

Still other features of my invention will best be understood by reference to the following description, taken in connection with the drawings, in which 1 `have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect. f

Fig. l schematically shows one embodiment of the invention,

Fig. 2 is a graphic representation of an ideal resonance curve of the discriminator network of Fig. 1,

Fig. 3 is a modification of the system of Fig. 1 wherein further circuit details are shown,

Fig. 4 presents an idealized graphical analysis of the functioning of the present system,

Fig. 5 shows a further embodiment of the invention applied to reproduction of frequency modulation-recorded impressions,

Fig. 6 schematically shows the invention applied to a modification of the system of Fig. 1,

Fig. 7 'illustrates an embodiment of the invention showing both condenser and magnetic reproducer elements,

Fig. 8 shows a modification ofv the circuit of Fig. '7,

Fig. 9 illustrates schematically the manner of embodying the invention in a frequency modulated carrier wave receiving system.

While the present invention is of wide utility,

as will be clear after a reading of lthe following description of .the various aspects and features of the invention, the embodiment of .the invention shown in Fig. 1 will be presented with a View to explaining the functioning of my novel discriminator-reproducer network. f AAs stated above., it has been proposed in the past -to reproduce sound records by employing a superaudible carrier which is frequency modulated by reactance' variations derived from the record impressions.' Accordingly, inFig. 1 I have schematically represented by the numeral I a superaudible oscillator network which is provided with the usual resonant tank circuit 2. The latter consists of a coil 3 and shunt condenser 4, and is tuned .to a predetermined normal oscillation frequency Fc. The predetermined frequency of the superaudible oscillator may be chosen from a wide range of frequency values depending upon the design of the other networks of the system. The oscillation frequency may be in the megacycle (mc.) range, or it may be in the lower kilocycle (kc.) values. By way of specific example, let fthe-assumed that the oscillator I has its tank circuit 2 tuned to a frequency of 20 kc. The frequency of ythe tank circuit 2 is modulated or deviated by virtue of reactance variations caused by the tracings of the needle or stylus through the record grooves.

By way of specific illustration, the numeral 5 denotes a record on a turntable 6. An electrostatic pickup device is schematically shown, and comprises a small condenser one of whose plates I is relatively fixed. The plate 'I is connected to the high alternating potential side oi tank cirn cuit 2. The mobile electrode 8 is indicated as grounded, and, therefore, is effectively connected to the grounded side of lthe oscillator tank circuit. The mobile electrode has operatively associated therewith a stylus which runs through .the record grooves. Hence, the capacitance of condenser 'I-8 will vary in accordance with the physical displacements of the stylus. These capacitance variations are translated into' frequency devia tions of tank circuit 2. The electrostatic pickup lated wave energy. The term discriminator used in the description and claims denotes a circuit which functions to produce an amplitude variation in the frequency modulated wave energy, which amplitude variation is representative of the frequency deviations of the wave energy. The latter wave energy is then capable of being rectified in order to provide the modulation signals for reproduction. This is true whether the sourceof the frequency modulated energy is directly responsive to a record, or whether a frequency modulation radio receiver is being employed.

According to my present invention, acoustic reproduction of the modulation signals occurs simultaneously with discrimination of the angle modulated wave energy. Stated differently, I have provided a discrimin-ator circuit which automatically and concurrently acoustically reproduces without the usual rectification .the amplitude modulated wave energy produced by discrimination. Reference to Fig. 1 will show that the FM wave energy, subsequent to amplification vat 9, is applied to a condenser C and coil L providing a parallel resonant circuit. The condenser C is actually an electrostatic .type of acoustic reproducer. Those skilled in the lart of sound reproduction are acquainted with the manner of Y designing an electrostatic type of reproducer, such device may be constructed in accordance with the teachings of my application Serial No. 414,305, led October 9, 1941, now Patent No. 2,376,456 issued May 22, 1945. However, the invention is not limited to this type of Idevice .as any other form of pickup maybe used, as long as it'l is capable of transforming the physical displacements of the stylus into reactance variations of the oscillator tank circuit.

The extent or range of frequency deviation with respect to Fc, the predetermined normal oscillator frequency, will be representative of the amplitude of the sounds which are recorded on record 5. while the rate of frequency deviation will be representative of the audio frequencies per se. I'he maximum frequency swing on either side of the mean frequency will depend also upon the design of the oscillator circuit. For example, the overall frequency swing or range may be some 10 kilocycles (kc). The frequency modulated oscillations may then be amplified as at 9. According. to my invention, Ithe amplified frequency modulated wave energy is subsequently treated in an entirely unconventional and novel manner.

In the prior art, it is usual to subject frequency modulated carrier wave energy to discrimination in order to translate the frequency modulated wave energy into corresponding amplitude moduas a condenser loud-speaker.

A .condenser loud-speaker is .a loud-speaker in which the mechanical forces result from electrostatic reactions. The plates of a condenser loudspeaker are relatively charged .due to polarizing voltage. The polarizing voltage may be as high as +200 volts or more. The mobile electrode of a condenser loud-speaker radiates the sound directly into the air. Provision must be made to keep the electrodes of the .condenser separated without at the same time incurring much mechanical stiffness. 'Io obtain an efficient system the mass of the mobile electrode is best kept low. The impedance of the amplifier .tube driving the condenser speaker is preferably made approximately equal to the capacitative impedance at the highest audio frequency to be reproduced so .that the capacity current drawn by the speaker will not cause excessive falling-off of voltage at the higher audio frequencies. It is an impor-tant and novel feature of my invention .that the modulated superaudible voltage applied to the condenser speaker C serves to provide polarizing voltage as required by the electrostatic speaker. A special or auxiliary polarizing voltage source is thereby eliminated.

The showing of the reproducer .construction in Fig. 1 is highly illustrative and schematic, since the particular type of reproducer is well known,

and my invention is not dependent upon any particular reproducer construction. In general, it comprises a metal backing plate I0 of wavy configuration so as to provide spaced peaks. The lead II connects the plate I0 to the lower end of .coil L. The condenser I2 establishes both the lower end of coil L and the plate'I 0 at ground potential for alternating current. The upper end of coil L is connected to the output electrode (not shown) ofthe amplifier 9. The +B terminal of the direct current supply source (not shown) is connected to the lower end of coil L. The second electrode of condenser C is provided by a thin metal sheet I3, as for example, tin or aluminum foil. It is obvious Ithat this metallic surface could be composed of sputtered or deposited gold or silver lof essentially molecular thickness as long as .con-

sheet vHI and ,electrode I only atgthe -peaks of the latter. The metallic or metallized sheet I3-is in contact With the dielectric sheet I4 at all points.

Hence, the metallic surface is rendered effectively elastic clue to the subjacent dielectric. As stated previously; the specific structure of the condenser speaker C is not part of my present invention; See, for example, U. S. Patent No. 1,644,387 to Kyle for a reproducer construction which is similar to that shown herein.

The' ideal discriminator characteristic of the circuit L--C is shown in Fig. 2. It is to be understood that the graphicrepresentation of Fig. 2 is purely illustrative. The curve in Fig. 2 is a single peak resonance curve Whose peak frequency is the resonant frequency of the circuitl which includes the coil L in shunt with the condenser C. The predetermined mean or center frequency Fe of the tank rcircuit 2 of the superaudible oscillator is off-tune with respect to the peak frequency of circuit L-C. The extent of off-tuning will be determined by the shape of the resonance curve. u

The peak frequency of the electrostatic reproducer circuit L--C is preferably so chosen that the mean frequency of the modulated Waves applied to the reproducer circuit will fall on a point on either flank of the resonance curve which has a substantial slope. Hence. it will be seen that the applied frequency modulated superaudible oscillations will be translated into correspondingv Such ampli-l amplitude modulated oscillations. tude modulated wave energy will be reproduced by the electrostatic reproducer C. Accordingly; my method generically contemplates the translation of angle modulated superaudible oscillations into corresponding amplitude modulated oscillations and concurrent reproduction of the audio modulation which exists on the translated wave energy,

In Fig. 3 I have shown specific circuit elements which may be employed in the schematic networks of Fig. 1. A morev detailed explanation of the operation of the condenser reproducer will be given in connection with Fig. 3. The electrostatic pickup device 8-,1, which constitutes a condenser, is shown. as having its fixed electrode l connected to the junction of the coil I6 and blocking condenser I1. The coil I6 and condenser 8--1' provide a resonant grid circuit for the oscillator'tube I3. The resonant grid circuit will norl mally be tuned to the predetermined mean frequency Fc. The plate circuit of tube I8 includes a resonant circuit I9 which is also tuned to the mean frequency Fc. It will, therefore, be recognized that Athe superaudible oscillator shown in Fig. 3, is of the well known tuned grid-tuned plate type. The variations in capacitance of the pickup device 1 8 in response to stylus motion cause frequency modulation of the oscillations produced in the oscillator circuit. Hence, there will be developed across the plate circuit I9 frequency mod--y ulated super'audible oscillation voltage. Thelatriez` wave employed in scribed in detail. The plate circuit of tube 2| includes a tuned circuit 22- which is resonated to.

the mean frequency Fc. Hence, across tuned circuit 22 there will be developed amplified frequency modulated superaudible oscillation voltage, This voltage may be further amplified in the output tube 2'5. The input grid of the latter is coupled by direct current blocking condenser 23 to the plate side of the resonant circuit 22. A suitable choke coil 24 connects the input grid of 'tube 25 to ground. The plate of tube I1 is connected to the +B terminal of the direct current supply source through the coil L which was described in connection with Fig. i. The condenser speaker C, schematically represented, is shown connected in parallel with coil L.

As stated previously, it is desirable to have the output impedance of tube 25 match the capacitative impedance of the condenser speaker C at the highest audio frequency to be reproduced. The circuit L-C acts as a high impedance in the plate circuit offtube 25. The tube 25 may be operated as a class A, class B or class C ampliiier. There are advantages in class C operation; for example, considerable power output could be secured even though tubes of relatively reduced dimensions were used. If desired, the power output stage may use a pair of-,tubes in push-pull relation. The bypass condenser I2 is connected from the lower end of coil L to ground, as in Fig. 1. Since the particular construction of the condenser reproducer C is no part of my invention, the reproducer is schematically represented in Fig. 3. 'I'he mobile electrode, or diaphragm, is denoted by numeral I3', While the numeral I0 designates the fixed electrode corresponding to electrode III of Fig. 1. Circuit L--C is tuned to the peak frequency of the curve shown inFig. 2. The frequency Fc Will fall on either flank of the curve; at about of resonance.

Before explaining the functioning of circuit L-C, let it be understood that the following analysis is equallyapplicable to an electromagnetically-operated device of the diaphragm type. The only requirement is that the original displacement of the diaphragm be secured in response to carrier Voltage. In any event the carthe system should be of superaudible frequency, and is required'to be'of an intensity capable of providing initial displacement of the diaphragm. Under these conditions, assumethe sound reproducing element is either the coil or condenser of the resonant discriminator circuit, and that Fe of the applied frequencyvariable waves falls on either flank of the discriminator resonance curve, The variations in frequency of the modulated Wave energy Will cause a change in polarizing voltage, and consequently the diaphragm displacement will change. If the variations in frequency of the FM energy applied to circuit I r-C occur at an audio frequency rate, sound will be emitted by the reproducer reactance element.

Another Way of looking at the action of vthe discriminator circuit L-C is as follows: Regardless of the polarity of the super-audible frequency current, the diaphragm elect-rode will be displaced in the same direction thereby-produc-4 I8., The remainder'of the ing second'harmonic output of the applied superaudible carrier frequency. This can bethought se) v willV result in acoustic output from the re producer element in step with carrier frequenc changes. f

- `In order more clearly to depict the functioning of the present system, attention is directed to the-curves A, B, C and D of Fig. 4. These curves are ideal representations, and are purely illustrative.- K'Cu'rve A shows a modulating audio Wave assumed to represent the recorded sound wave. This curve can represent the change of capacity of-the -FM pickup 1-8 which produces changes' in oscillator frequency as I*shown by curve B. The curve A' could also 'represent the voltage output of piezo-electric `crystal 4I) of Fig.'6. The curve pictorially shows the relative values of the modulating and modulated frequencies. Curve B represents the frequency modulated wave resulting fromY variation of the pickup device 1 8 in response to the variations of curve A. vThe oscillator voltageduring modulation would generally follovi7 the wave form of curve B. The amplitude is constant, while the frequency is variable. vThe effect 'of' circuit L-C on the 'wave form of curve Bl is depicted'in curve C. It will be observed that the frequency-variable wave now has variable amplitude as well. It will further be seen that the variable amplitude of curve C follows the variation of the modulating audio wave of curve A. Finally, curve D illustrates the pullonthediaphragm electrode (I3 in Fig. 1 or I3' in Fig;v 3) of' condenserreproducer C. Note that the pull is always in one direction, at double the carrierffr'equency and of a variable amplitude `determined by the voltage across L-C as shown byl curve C. Due to the audible variations in pullon the diaphragm the sound emitted by the'diaphragm 'will be a' reproduction of the recorded sound.v Curve D is more closely spaced than the prior curve C, because it represents the pull on the diaphragm, and this occurs at double theA carrier frequency shown in curves B and C.

' The frequency'range of the superaudible oscillator is preferably Vbetween 15 and 30 kc., although notnecessarily limited thereto. Generally, the oscillator frequency should be several times 'higher than the highest audio frequency which it v'is desired to reproduce. 'For an oscillator having a frequency of 15 kc., it is most likely that frequency deviations up to a maximum of plus orminusA 2 'to 3 kc. should be suilicient. At an oscillator frequency of 30 kc. it might be desirable'to employ frequency deviations up to al maximum of plus or minus 10 kc. These frequency values are not intended to be limiting in nature, but are merely illustrative.

YIt has been previously stated that one of the main features of my invention is to utilize the appliedicarriervoltage as the polarizing voltage for the condenser. The following explanation is not necessarily exclusive of other theoretical explanations of the demonstrated fact that no auxiliary'direc't current `polarizing voltage is needed in my condenser reproducer. If like voltages and -l-) are applied to electrodes I and I3, there will, of course, be set up a mechanical force tending to push or repel the electrodes from each other. 4The electrodes are attracted to each other uponk the application of unlike 'voltages 8 (3+ and thereto. 'In circuit L-C there willappear across coil L, and'consequently across C, an alternating voltage having the frequencyFc.

Since the opposite ends of coil L must 'havefoppositepolarity potentials, the opposite electrodes of C must be of opposite'polarityl thereby causting attraction between them.

This gives therequired initial displacement of the speaker` diaphragm electrode I3. The amount' ofdisplacement increases as the voltage across` coil L increases. 1The carrier voltage of mean frequency Fc will produce a displacement of-.the-

diaphragm for each half cycle, since the only. requirement for the attractionv between the diaphragm I3 and fixed electrode I0 is that opposite polarity potentials be applied. Thus, as far.

as the diaphragm is concerned the pull on it will occur at double the carrier frequency Fc. Since Fc is fundamentally above audibility the variation in pull on thediaphragm at 2Fc cannot be heard. Therefore, the same effect is produced, asfar asv the electrostatic speaker is concerned,

a's though direct current voltage were applied to the electrodes in the well known manner.

Having initially displaced the diaphragm I3 against the resilient -sheet or darn I 4, there is created a restoring-force due to the stressed dam. This force tends to return -the diaphragm I3 to its original. normal position'. If we now change the magnitude of the polarizing voltage, the sound output must result. This acoustic output occurs as the attraction between the electrodes I0 and I3 varies with the applied voltage, and the variation Will occur'at an audio frequency rate correspondingv to the modulating frequency yapplied to the carrier wave.

There may be some improvement in acoustic output by operating at point Fc on the curve of Fig. 2 rrather than at point Fc on the high frequency ilank ofthe curve. It has previouslybeen `explained .that as the voltage applied to speaker electrodes increases, the pull on the diaphragm increases. As the diaphragm is displaced in this fashion by a change in the instantaneous value of the applied modulated energy, the capacity of condenser-C .increases thereby to lower the fre.- quency ofL-C.. y.This causesa.further'increase in voltage and further displacement of electrode I3. Obviously this cannot'go on very long, since there is not necessarily a linear relation between capacity change, displacement of the diaphragm and the polarizing voltage. `Some of the eiect is present, however. Similarly as the instantaneous frequency is Lshifted downward by modulation, the Voltagenacross condenser C decreases. This causes decrease in displacement of electrode I3, with concurrent upward increase of the resonant frequency of L-C thereby causing a further decrease in voltage across condenser C. Thus, tuning the circuit L-C sc .that Fc falls on the low frequency flank of the resonance curve of circuit LC, should result in some increase in output.

' Assume, now, that the operating point is at Fa'.y This means that Vthe normal Vfrequency of oscillator' is 'Fc'. If the instantaneous frequency of the FM voltage applied' to 'L-C is above Fe then a'decrease ofv outputvoltage occurs. The' displacement of lthe `diaphragm will decrease thereby causinga decrease in effective capacity of condenser C.: This will, in turn, cause circuit L--C to tune to a higher frequency resulting in' stillless voltage and displacement. condition occurs when. the modulated Lenergy lhas an instantaneous:frequency less than Fe'.; There A similar is thus obtained less acoustic output'from-an overall viewpoint when operating at Fc than at Fc on the resonance curve of circuit L-fC.

My invention has many possibilities. The pickup device need not be electrostatic, but may be of the variable inductance type. The records used may be of the conventional laterally cut type; the hill and dale type; or nlm recordings of either amplitude-variable or frequency-variable types. Moreover, the records may be of the frequency modulation type, wherein frequency modulation signals are directly recorded. In general, any transducer device can be employed in conjunction with a superaudible oscillator to vary the frequency of oscillations thereof, and the resulting frequency modulated oscillations after amplifica.- tion can be reproduced in a discriminator-reproducer circuit L-C as described herein. By the term transducer I include a microphone, a pickup device whether of the electrostatic, magnetic or piezo-electric crystal types or any other mechanism capable of translating acoustic or recorded waves into electric currents of audio frequency.

In Fig. I have shown a modincation of the system or 3 to the extent that the amplifier 25 is supplied with amplied frequency modulated signals from a piezo-electric crystal pickup device. The record in this form of the invention is generally of the type disclosed by W. van B. Roberts in his application Serial No. 369,829, led December 12, 1940. The record 5 has cut or engraved in its face impressions corresponding to FM signals whose mean carrier frequency is superaudible. Here, again, the recorded FM signals may have a mean frequency chosen from a range of to 30 kc., with a maximum deviation range from 2 to l0 kc. The recorded FM signals are converted into FM currents by the crystal pickup 3B. The latter is schematically represented, since it is very Well known to those skilled in the art of reproducing recordedrsound.. The FM currents produced at the output terminals of pickup 3l) are amplified in one or more stages of amplifica-tion 3l and 3i'. 3lv are each tuned to a predetermined normal frequency Fc. 'Ihe aforesaid mean carrier frequency is Fc. The amplied FM signals applied to amplifier are reproduced by the conjoint discrimination and reproduction action of L--C as previously described. l

In Fig. 6` I have shown a dirferent method of producing the FM signal energy for application to the circuit L-C. In this system the record 5 is a standard amplitude vrecord of the laterally-cut type or of the hill and dale type. The pickup device 43 is a suitable form of piezo-electric crystal pickup, as in the case of Fig. 5, and feeds its audio frequency output current to the input terminals of a superaudible frequency oscillator ci. rihe latter is normally tuned to produce oscillations ci a frequency Fc. The enect of the audio input current is to vary the frequency of oscillation of the oscillator di. Any formof relaxation oscillator. such as the well known resistance-capacity oscillator or thel multi-vibrator oscillator, may be used at I prefer tc employ the R'C type of oscillator disclosed and claimed for the same function in my application Serial No. 463,349, nled Cctcber 26, 1942. The last-named application explains the manner in which the audio frequencycurrents, derived from a sound record, may be used to frequency modulate the oscillatory output current of-an R-C oscillator of the-form disclosed-therein.; .f

The ampliners 3| and The resulting FM currents may be amplified. as atamplifier 42 tuned to the mean frequency Fc, and then applied to the output amplifier 3. The latter has the circuit L-C in its plate circuit exactly as described in Fig. 3. The action of circuit L-C is the same as previously explained, and the audio frequency current repro-duced by condenser reproducer C-is emitted as sound.

As stated previcusiy,. the present invention is applicable to a resilient diaphragm type of electromagneticaliy-operated device. The diaphragm may have sufficient inherent resiliency to spring back, or restore, into normalA horizontal position, orA any well known and suitable mechanical restoring device may be used. The only requirement is that the original displacement of the diaphragm from normal position be obtained by means of carrier-derived voltage. The ampliiied voltage at carrier frequency Fc (applied to the driving coil) should be sufiiciently high to provide the necessary polarization and initial', minimum or threshold displacement of the diaphragm. In Fig. 7 I have shown the output amplier 5o of a system fed with FM waves produced in any of the ways shown in Figs. 1, 3, 5 or 6. The amplifier tube 50 has the circuit L--C arranged in its plate circuit asv previously explained. The condenser C is a reproducer which has a high audio frequency respons@ characteristic. In other words it is of the so-called tweeter type. The coil L is the primary winding of a transformer 5l Whose secondary winding 52 is arranged in a closed circuit with a low impedance coil 53. The coil 53 isA provided with a diaphragm 5s which is initially displaced or polarized by virtue of the carrierderived voltage developed across coil 53. Coil 53 may be provided with a core 53 of magnetic material readily adapted to providea magnetic pull on diaphragm 5s against the illustrative leaf spring restoring device 55. The magnetic reproducer 53-54 is of the low audio frequency response, or so-called woofen type.

This form of dual reproducer system operates in a manner similar to the prior circuits LC. In the present case the primary circuit of transformer 5l is tuned t0 the peak frequency of the curve shown in 2. The mean frequency of the applied FM signals falls on either flank of the curve as previously explained. The-reproducer C functions in response to the envelo-pe of the rectified amplitude-variable wave, as depicted in Fig. 4 by curve D. The variable amplitude carrier currents flowing through primary winding L will induce similar currents in secondary winding These variable amplitude currents will vary the position of the initially-displaced diaphragm Dotted line A' showsl the displaced positionl of the diaphragm at the peaks of the curve D in Fig. e, while dotted line B shows the minimum signal, or threshold, displaced position at the valleys of curve D of Fig. 4. It will be seen that the diaphragm is pulled towards the-magnetic core in response to the minimum signals flowing-through coil 53. As the signal current modulatesv therestoring element will be caused to vary-the diaphragm position. i

In Fig. 8 there is shown a modication of the dual reproducer system of Fig. '7. Here the FM signal current output of tube Bil-flows through the-discriminator circuit C',L. Condenser- Cf isl a reproducerA whose mobile or diaphragm electrode-is designated by numeral I3', and the static electrode is l0. Coil L is a high impedance coil andmaybevprovided with corei3?4 l1 and initially displaces diaphrag-m 54 due to the carrier-derived polarizing voltage. Circuit C- L is tuned to the peak frequency of a resonance curve (Fig. 2) upon either of whose flanks may fall Fc, the mean frequency of FM signals applied to amplifier 50. Of course, the reproducers may have different, such as opposite or complementary, frequency response characteristics. The plate of tube 50 is connected to the +B terminal through isolation coil 6I, and direct current blocking condenser 60 connects the plate to electrode I3'. Additionally, the condenser C' may be a normal condenser and non-reproducing,

the coil L and its diaphragm acting then as the reproducer.

To recapitulate the operation of the systems of Figs. 7 and 8, the circuit L-C performs the same as previously described. In Fig. 7 the coil L forms the primary of an output transformer 5| which matches the speaker coil 53 to the output of tube 50. Thus, as the voltage across coil L varies with modulation there will be a variation in voltage across coil 53. Consequently there will occur a change in displacement of diaphragm 54. The particular advantage of the system of Fig. 7 resides in the fact that diaphragm 54 can reproduce low frequencies, whereas it is diiiicult to obtain satisfactory reproduction of low audio frequencies from an electrostatic speaker. Coil 53 could be the voice coil of a loudspeaker, and serves to displace the diaphragm 54 initially under influence of the carrier voltage.

In Fig. 8 the set-up is somewhat different. In this instance L-C' performs normally. Coil Lf is, however, magnetically coupled to diaphragm 54' so that displacement of the latter varies with changes in voltage across coil L. v'I'his has been explained in connection with Fig. 7. Again, the purpose of diaphragm 54 is to reproduce the lower audio frequencies while condenser C reproduces the higher audio frequencies. It is to be clearly understood that the inherent restoring characteristic of a springy diaphragm may be used in either Figs. 7 and 8. Hence, in Fig. 8 I omit the leaf spring restoring device of Fig. 7.

My invention is applicable to the reception of FM broadcast waves in the presently-assigned 42-50 megacycle (mc.) band. Such waves have a considerable deviation range. For example. at present an FM transmitter in the 42`50 mc. band is permitted a maximum frequency deviation up to '.75 kilocycles (kc.) on either side of the transmitter normal or carrier frequency. Assuming superbeterodyne reception of such FM waves at the receiver system shown in Fig. 9. those skilled in the art are fully aware of the fact that the mean frequency of the received signals is reduced to a lower frequency value, say of the order of 4.3 me., without changing the range in absolute figures of the frequency swing of the intermediate frequency (I. F.) energy. Hence, I. F. amplifier El) is to b-e understood as having its inout terminals coupled to the I. F. output circuit ci a prior converter. The I. F. amplifier passes FM energy whose mean frequency is 4.3 mc.. and whose frequency swing may cover a rance of plus or minus 75 kc.

The discriminator-reproducer circuit L-C is arranged in the output circuit of the last tube of the system. It is not practicable to apply the FM current output of I. F. amplifier 60 to circuit L-C. The mean frequency of 4.3 mc. is too high, and the i-75 kc. swing is too great for circuit; L--C to handle. Hence, I utilize cascaded frequency dividers 6| and 62 to reduce concurrently the mean frequency and frequency swing to more practicable values of 172 kc. and 5 i3 kc. respectively. This method is generally in accordance with the teachings of M. G. Crosby in his U. S. Patents 2,230,231 and 2,230,232 granted February 4, 1931. The frequency divider 6I reduces the I. F. energy by a factor of 5, and divider 62 reduces the FM energy output of divider 6I by a factor of 5.

Each of dividers 6l and 62 may, if desired, be constructed in accordance with the teachings of G. L. Beers in application Serial No. 430,588, filed February 12, 1942, patented Aug. 22, 1944, as U. S. Patent No. 2,356,201. Beers shows a locked-in oscillator whose input terminals has FM signal energy applied thereto (I. F. stage), and in the output circuit there is produced FM signal energy Whose mean frequency and frequency swing have been reduced by a like factor. Of course, any other form of frequency divider may be used at 6l and 62. It is to be clearly understood, however, that additional frequency division may be employed to reduce the 172 kc. mean frequency, or heterodyning action may be used for the same purpose if the deviation is not to be affected. Further, the receiving system may operate in any FM transmission band. Again, either of the reproducer systems of Figs. 7 and 8 may be employed in place of that shown in Fig. 9. The functioning of the circuit L-C in Fig. 9 will be similar to that described in Figs. 1 and 3. Circuit L-C is tuned to a frequency above or below 172 kc., and as depicted in Fig. 2. The carrier-derived voltage functions as polarizing voltage in this case as well.

While I have indicated and described several systems for carrying my invention into effect, 40 it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scopel of my invention. 45 WhatIclaimis:

1. In a system for reproducing frequency modulated carrier waves, a resonant descriminator circuit including as the capacitative element thereof an electrostatic reproducer, and

said discriminator circuit being tuned to a frequency which is substantially off-resonance with respect to the mean frequency of frequency modulated waves applied to the discriminator circuit.

2. In a method of reproducing sound records wherein angle modulation of a superaudible wave is employed to represent currents corresponding to sounds to be reproduced by a loudspeaker; the improved step comprising the simultaneous translation of angle modulated superaudible Wave energy into corresponding amplitude modulated wave energy and reproduction of the modulation existing on the latter Wave energy in a single discriminator circuit.

05 3. A frequency responsive network, adapted to have angle modulated superaudible waves applied thereto, comprising a coil and a condenser loudspeaker, said condenser loudspeaker being adapted directly to convert the modulation on the aplied waves into sound, and said coil and condenser being tuned substantially olf-resonance relative to the mean frequency of applied modulated Waves.

4. In combination with a source of frequency modulated oscillations, a. condenser reproducer,

and a circuit connecting the reproducer to said source for reproduction of the modulated oscillations; the improvement comprising said circuit including a coil which 4forms a tuned frequency discriminatcr circuit with the condenser reproducer, said coil and condenser being tuned to a frequency sufficiently different .from the mean frequency of the modulated oscillations to secure frequency discrimination.

5. In a system for receiving frequency modulated carrier waves, a resonant discriminatcr circuit including as the active capacitative reactance element thereof an electrostatic reproducer, and said discriminator circuit being tuned to a frequency which is substantially off-resonance with respect to the mean frequency of frequency modulated carrier voltages applied to the discriminatcr circuit.

6. In a method of reproducing sound wherein angle modulation of a superaudible Wave is employed to represent modulation voltages corresponding to sounds to be reproduced; the irnprovement consisting of the concurrent translation of angle modulated superaudible wave energy into corresponding amplitude modulated Wave energy and reproduction of the modulation existing on the latter wave energy in a simple tuned circuit off-resonance with said wave frequency.

7. A frequency responsive network comprising a coil and a condenser, at least one of the two reactances being adapted to convert modulation voltages into sound, and said coil and condenser being substantially off-resonance relative to a desired mean frequency of frequency modulated carrier waves adapted to be applied to said network.

8. In combination with a source of angle modulated oscillations, a condenser reproducer, a circuit coupled to said source and including a coil which forms a tuned discriminatcr circuit with the condenser reproducer, said coil and condenser being tuned to a frequency substantially di'erent from the mean frequency of the modulated oscillations to secure discrimination.

9. In combination with a reactive transducer, a superaudible oscillator, means for varying the frequency of the oscillator in accordance with the transducer output energy, means for reproducing the frequency modulated oscillations without rectification, said means comprising a discriminatcr circuit substantially off-tune with the mean frequency of the oscillations, and the capacitative component of the discriminatcr circuit being an electrostatic reproducer,

10. In combination in a receiver of frequency modulated carrier waves having a. high mean frequency and a relatively high frequency deviation ratio, and at least one frequency divider for reducing the said mean frequency and deviation ratio by a like factor; the improvement which includes means for reproducing the divided Wave energy without rectification, said last means comprising a tuned discriminatcr circuit composed of at least two reactive elements of opposite sign, at least one of said reactive elements being a modulation reproducer.

11. In a system for reproducing frequency modulated carrier Waves derived from sound records, a discriminatcr circuit including as a resonating reactive element thereof a transducer, and said discriminatcr circuit being tuned to a. frequency which is substantially off-resonance with respect 14 to the mean frequency of frequency modulated carrier waves applied thereto.

12. A method of reproducing sound records whose impressions are provided by angle modulation o-f a superaudible wave, comprising applying the angle modulated wave energy to a resonant circuit tuned substantially off-resonance relative to said Wave frequency, and providing in said resonant circuit the simultaneous translation of the angle modulated superaudible wave energy into corresponding amplitude modulated Wave energy and reproduction of the modulation existing on the latter wave energy.

13. A frequency responsive network, adapted to have angle modulated superaudible waves applied thereto, comprising a coil and a condenser, one of said latter reactive elements being Ia transducer, and said coil and condenser being tuned substantially off-resonance relative to the mean frequency of applied waves.

14. In combination with a source of angle modulated oscillations, a condenser reproducer, an amplifier Ycircuit connecting the reproducer to said source for reproduction of the modulated oscillations, a tuned discriminator circuit consisting of a coil in shunt with the condenser reproducer, said coil and condenser being tuned to a frequency sufficiently different from the mean frequency of the modulated oscillations to secure discrimination.

15. In combination with a reactive transducer, a. superaudible oscillator, means for varying the frequency of the oscillator in accordance with the transducer output energy, a discriminatcr circuit substantially off-tune with the mean frequency of the oscillations, and the capacitative component of the discrminator circuit being an electrostatic reproducer,

16. A frequency discriminatcr circuit consisting of two reactive components of opposite reactive sign related in magnitude to tune to a predetermined peak frequency, and at least one of the reactive components being a transducer.

17. A discriminatcr circuit including as an element thereof an electrostatic reproducer, and

said discriminatcr circuit being tuned to a frequency which is adapted to be substantially cifresonance with respect to the mean frequency of modulated carrier waves applied to the circuit.

18. In combination with a source of angle modulated oscillations, a condenser reproducer, a circuit connecting the reproducer to said source for reproduction of the modulated oscillations, said circuit including a coilV which forms a tuned discriminator circuit with the condenser reproducer,

means for polarizing said condenser with voltage derived from the oscillations, said coil and condenser being tuned to a frequency substantially different from the mean frequency of the modulated oscillations to secure discrimination.

19. In combination with a reactive transducer, a superaudible oscillatc-r, means for varying the frequency of the oscillator` in accordance with the transducer output energy, means for reproducing the frequency modulated oscillations without rectification, said means comprising a discriminatcr circuit substantially off -tune with the mean frequency of the oscillations, ther capacitative component of the discriminatcr circuit being an electrostatic reproducer, and means responsive to voltage derived from said frequency modulated oscillations for polarizing said electrostatic reproducer.

CHESTER M. SINNETT. 

